/Engine/Source/Runtime/GeometryCore/Public/Implicit/SweepingMeshSDF.h
Robust test of (X0,Y0) in the triangle (X1,Y1)-(X2,Y2)-(X3,Y3) if true is returned, the barycentric coordinates are set in A,B,C.
static bool PointInTriangle2d ( double X0, double Y0, double X1, double Y1, double X2, double Y2, double X3, double Y3, double & A, double & B, double & C )